This invention relates to a turbo vacuum pump in which an outlet port pressure and atmospheric pressure are the same and, more particularly, to a vacuum pump suitable for generating a clean vacuum for use in equipments for manufacture of food, pharmaceuticals, and the like.
In a conventional turbo vacuum pump, an axial flow blade having excellent discharge performance in the molecular flow region is often used. The other hand, in recent years, a turbo vacuum pump has been developed which can provide a high compression ratio in the viscous flow region and has peripheral-flow impellers formed in multiple stages. An example of this is described in Japanese Patent Unexamined Publication No. 2-264196. The turbo vacuum pump described therein can discharge air into atmosphere but it has a drawback that disassembly of a pump rotor is not easy since the pump rotor and a pump stator are staggered.
Also, a turbo vacuum pump has been proposed in which pressure level of an outlet port and atmospheric pressure are the same, an example of which is described in Japanese Patent Unexamined Publication No. 3-7039 (corresponding to U.S. Pat. No. 4,668,160). In the turbo vacuum pump, a stator forming an air path protrudes between rotor blades. Therefore, the stator must have structure which is dividable in an axial direction into two halves. As a result, the number of parts of the stator increases. Accordingly, when the stator is assembled, dimensional tolerances of the parts are accumulated and it becomes difficult to control dimentions of thin clearances between the stator and the rotor and to obtain the thin clearances. It is difficult to obtain a desired pump performance.